Influence of Ge Lone Pairs on the Elasticity and Thermal Conductivity of GeSe–AgBiSe2 Alloys

Chemical design is a compelling strategy to improve the performance of energy materials but requires a comprehensive understanding of how structure and chemistry affect materials properties. In this work, we investigate the structural, elastic, and thermal behavior of the (GeSe)1−x−(AgBiSe2)x system to shed light on the respective contributions of chemistry and crystal structure to thermal transport. In this system, progressive AgBiSe2 alloying transforms the room-temperature structure from orthorhombic to rhombohedral to cubic rock-salt, with the latter being the thermodynamically stable phase across the entire compositional range at a high temperature. Within a given structure type, alloying progressively increases point-defect phonon scattering and decreases the speed of sound, thus reducing the lattice thermal conductivity. However, an anomalous increase in thermal conductivity is noticed upon the transition from the rhombohedral to the cubic phase, associated with a large step-like increase in the elastic moduli. Density functional theory calculations suggest this change in stiffness is due to the tendency of Ge to exhibit strongly expressed lone-pair orbitals. The lone pairs switch from fully oriented in the rhombohedral phase to randomly oriented in the cubic phase, leading to an overall bond stiffening. Supporting this argument, increased AgBiSe2 content, and thus weakened lone-pair expression, leads to a less pronounced stiffening at the phase transition. These findings help improve our understanding of the intrinsic properties of this system and the design of rock-salt chalcogenides with tailored thermal properties.